The present invention relates to wireless mesh network systems. In particular, the invention relates to wireless mesh network systems having an improved performance through the use of selected nodes with different communication characteristics.
In many industrial settings, control systems are used to monitor and control inventories, processes, and the like. Often, such control systems have a centralized control room with a host computer that communicates with field devices that are separated or geographically removed from the control room.
Generally, each field device includes a transducer, which may generate an output signal based on a physical input or generate a physical output based on an input signal. Types of transducers used in field devices include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow sensors, positioners, actuators, solenoids, indicators, and the like. Traditionally, analog field devices have been connected to the process subsystem and the control room by two-wire twisted-pair current loops, with each device connected to the control room by a single two-wire twisted pair loop. Typically, a voltage differential is maintained between the two wires of approximately 20 to 25 volts, and a current between 4 and 20 milliamps (mA) runs through the loop. An analog field device transmits a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. An analog field device that performs an action under the control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the ports of the process subsystem under the control of the controller.
While historically field devices were capable of performing only one function, more recently hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. The Highway Addressable Remote Transducer (HART) protocol superimposes a digital carrier signal on the current loop signal. The digital carrier signal can be used to send secondary and diagnostic information. Examples of information provided over the carrier signal include primary and secondary process variables, diagnostic information (such as sensor diagnostics, device diagnostics, wiring diagnostics, process diagnostics, and the like), operating temperatures, sensor temperature, calibration data, device ID numbers, configuration information, and so on. Accordingly, a single field device may have a variety of input and output variables and may implement a variety of functions.
Another approach uses a digital communication bus to connect multiple field devices to the host in the control room. Examples of digital communication protocols used with field devices connected to a digital bus include Foundation Fieldbus, Profibus, Modbus, and DeviceNet. Two way digital communication of messages between a host computer and multiple field devices can be provided over the same two-wire path that supplies power to the field devices.
Typically, remote applications have been added to a control system by running very long homerun cables from the control room to the remote application. If the remote application is, for example, a half of a mile away, the costs involved in running such a long cable can be high. If multiple homerun cables have to be run to the remote application, the costs become even higher. Wireless communication offers a desirable alternative, and wireless mesh networks have been proposed for use in industrial process control systems. However, to minimize costs, it is also desirable to maintain existing control systems and communication protocols, to reduce the costs associated with changing existing systems to accommodate the wireless communication.
In wireless mesh network systems designed for low power sensor/actuator based applications, many devices in the network must be powered by long life batteries or by low power energy scavenging power sources. Power outlets, such as 120 VAC utilities, are not typically located nearby or may not be permitted into the hazardous locations where the instrumentation (sensors and actuators) must be located without incurring significant installation expense. The economic need for low installation cost drives the need for battery powered devices communicating as part of a wireless mesh network. Effective utilization of a limited power source, such as a primary cell battery which cannot be recharged, is vital for a properly functioning wireless device. Batteries are expected to last more than 5 years and preferably as long as the life of the product.
In a true wireless mesh network, each device must be capable of routing messages for itself as well as other devices in the mesh network (so called mesh to the edge). The concept of messages hopping from radio to radio through the network is beneficial because lower power radios can be used and yet the mesh network can span a significant physical area delivering messages from one end to the other. High power radios are not needed as in a point-to-point system which employs remote devices talking directly to a distant centralized base station.
A mesh network allows for the formation of alternate paths for messaging between devices and between devices and a data collector or a bridge or gateway to some higher level higher speed data bus. Having alternate redundant paths for messages enhances data reliability by ensuring there is at least one path for messages to use even if another path becomes blocked or degraded due to environmental influences or interference.
Some mesh network protocols are deterministically routed such that every node has an assigned parent and at least one alternate parent. In the hierarchy of the mesh network, much as in a human family, parents have children, children have grandchildren, and so on. Each node relays the messages for their descendants through the network to some final destination such as a gateway. The parenting nodes may be battery-powered or limited-energy powered devices. The more descendants a node has, the more traffic it must route, which in turn directly increases its own power consumption and diminishes its battery life.
Normally, all nodes in the mesh network employ radios (RF transceivers) of the same quality each having the same link budget (Tx power-Rx sensitivity). To bridge long distances, many nodes are sometimes required to relay messages. In an industrial setting, it may be physically impossible or environmentally prohibited to install radios in certain locations to make these links. An improved method for using wireless mesh networks in an industrial setting is needed.